標題：Batesian mimicry promotes pre- and postmating isolation in a snake mimicry complex.

摘要

We evaluated whether Batesian mimicry promotes early-stage reproductive isolation. Many Batesian mimics occur not only in sympatry with their model (as expected), but also in allopatry. As a consequence of local adaptation within both sympatry (where mimetic traits are favored) and allopatry (where nonmimetic traits are favored), divergent, predator-mediated natural selection should disfavor immigrants between these selective environments as well as any between-environment hybrids. This selection might form the basis for both pre- and postmating isolation, respectively.We tested for such selection in a snake mimicry complex by placing clay replicas of sympatric, allopatric, or hybrid phenotypes in both sympatry and allopatry and measuring predation attempts. As predicted, replicas with immigrant phenotypes were disfavored in both selective environments. Replicas with hybrid phenotypes were also disfavored, but only in a region of sympatry where previous studies have detected strong selection favoring precise mimicry. By fostering immigrant inviability and ecologically dependent selection against hybrids (at least in some habitats), Batesian mimicry might therefore promote reproductive isolation. Thus, although Batesian mimicry has long been viewed as a mechanism for convergent evolution, it might play an underappreciated role in fueling divergent evolution and possibly even the evolution of reproductive isolation and speciation.

穆氏擬態的毒蝶 (Heliconius spp.)因為其環境的壓力（天擇）與自身的擇偶偏好（性擇），共同造就其不同區域的多態型，一些科學家認為不同的型態區隔或許可以是生殖隔離乃至種化的起源，也從早期的研究中找到部分的證據支持這個理論。然而因為擬態群特性的緣故，地理型態區隔不明顯等等，貝氏擬態或許也有一樣的現象，但支持的證據卻少之又少。

猩紅王蛇 (scarlet king snake, Lampropeltis elapsoides)與珊瑚蛇(coral snake, Micrurus fulvius)是新北區東南方著名的脊椎動物的貝氏擬態案例，可是自從被發現以來爭議不斷，主要是因為早期對於不太精確的擬態斑紋，以及有點無法解釋的分佈產生的疑惑，而近十年對於這個擬態群的研究我們對於這個擬態群又更瞭解了一些，包含猩紅王蛇的分佈其實比起珊瑚蛇來的廣，兩者分佈不重疊的區域擬態就就不精確，而造成這種分佈的原因可能來自廣域性的捕食者（猛禽）等等。這時候就產生疑問了，如果分佈共域/異域的型態不一樣，那有沒有可能像毒蝶那樣，有機會產生生殖隔離呢？

這個研究簡單的討論了這個問題，作者假設如果不同型態的族群無法進入不同的區域的話，不同型態的個體根本沒辦法遇見彼此，就會發生生殖前隔離(premating isolation)；而若是兩種型態在雜交帶產下中間型的個體，捕食者（天擇）不知道這是什麼就把牠吃了，那麼透過雜交帶的基因傳遞功能失效，就會產生生殖後隔離(postmating isolaiton)。作者用黏土做成三種不同的型態，分別是精確型/雜交型/不精確型，分別把這些型交叉測試，譬如把精確型放到異域的區域裡或者相反，測試這些型態的被攻擊率。

結果如同預期，不同型態在不同區域的確被攻擊的比率比較高，可是雜交型只有在共域的沙丘環境上被攻擊的比率比較高，其他環境統計上不顯著。因此作者認為，貝氏擬態的確有可能造成族群間的隔離（對雜交型來說，至少某些環境遭遇比較強的天擇壓力），進而造成生殖隔離乃至種化。

至於那個黏土模型.....好像挺微妙的？下次看作者會不會改成3D列印，搞不好做出來結果有差。

從這來的http://www.nhm.ku.edu/rbrown/PastHerpsOMonth.htm

這是一個做兩生類親緣關係與分類的研究室所拍到的照片，來自Department of Ecology and Evolutionary Biology, University of Kansas的Dr. Rafe Brown所拍攝的，在其網頁上註記其擬態關係。

挺有趣的，我是覺得不太像啦，不論是size或是pattern，再者，鱗翅目很多長這樣的東西，但跟珊瑚蛇不共域，第三，珊瑚蛇的擬態有斑紋動態的問題，但鱗翅目幼蟲不太會有這樣的行為，所以這個是否擬態大概要再討論。

原文標題：High-model abundance may permit the gradual evolution of Batesian mimicry: an experimental test

Kikuchi DW & Pfennig DW. 2009. High-model abundance may permit the gradual evolution of Batesian mimicry: an experimental test. Proc. R. Soc. B. in press. [page]

[摘要]

In Batesian mimicry, a harmless species (the ‘mimic’) resembles a dangerous species (the ‘model’) and is thus protected from predators. It is often assumed that the mimetic phenotype evolves from a cryptic phenotype, but it is unclear how a population can transition through intermediate phenotypes; such intermediates may receive neither the benefits of crypsis nor mimicry. Here, we ask if selection against intermediates weakens with increasing model abundance. We also ask if mimicry has evolved from cryptic phenotypes in a mimetic clade. We first present an ancestral character-state reconstruction showing that mimicry of a coral snake (Micrurus fulvius) by the scarlet kingsnake (Lampropeltis elapsoides) evolved from a cryptic phenotype. We then evaluate predation rates on intermediate phenotypes relative to cryptic and mimetic phenotypes under conditions of both high- and low-model abundances. Our results indicate that where coral snakes are rare, intermediate phenotypes are attacked more often than cryptic and mimetic phenotypes, indicating the presence of an adaptive valley. However, where coral snakes are abundant, intermediate phenotypes are not attacked more frequently, resulting in an adaptive landscape without a valley. Thus, high-model abundance may facilitate the evolution of Batesian mimicry.

這篇文章主要在談貝氏擬態的擬態型演化理論。在擬態生物學中牽扯到斑紋變化的主要有兩個理論，一個是Fisher (1930)所提出的two-step theory，另一個則是Fisher (1927)基於天擇說與穆氏擬態的特徵漸變理論，也就是特徵的變化是由環境逐漸淘汰所形成的，但這個理論在擬態生物學中之所以不被認為是一個常態，主要原因是擬態所遭受的天擇壓力十分強大，只要性狀有變異就可能遭受淘汰，因此若是一個物種無論從cryptic，或是從另一個擬態群要改變斑紋加入另一個擬態群，勢必要經歷的是斑紋從不相似到相似，而這個過程中會被捕食者所選擇。

珊瑚蛇擬態被認為是貝氏擬態的經典例子，在貝氏擬態的演化歷程中，被預測的是斑紋會快速的演變，也就是Fisher(1930)所提出的two-step theory較符合貝氏擬態的演化歷程，但此篇研究提供了另一個證據，也就是在貝氏擬態的前提下，仍然可以逐漸的改變顏色組合逐漸逼近model，但是必須在model豐度高的時候才成立，這篇研究經由phylogeny與野外的實驗來測試這件事情，結果都支持這個擬態群的顏色組變化是逐漸變化的，也就是不太像的mimic在變化時受到model豐度的保護，讓捕食者只要有一點像那個不好吃的東西就不吃，而在model豐度較低的地方，捕食者沒有那麼高頻度的遇到model，mimic只要稍有不像就可能遭受攻擊，phylogeny的結果則顯示整體的演化方向是cryptic->intermideately->precisely。

這篇文章提供一個擬態斑紋的演化機制，並且在某種程度上支持了frequency-dependent的論點，並且與這個研究群先前以族群遺傳做出來的結果一致，顯示擬態斑紋的變化可受到族群量的直接影響。